1. Field of the Invention
The field of art to which this invention pertains includes cargo container handling devices and more specifically an interlocking system for the twist locks mounted on the ends of a lifting beam of a container lifting spreader frame.
2. Description of the Prior Art
Large fully enclosed cargo containers in standardized lengths have become a staple of that part of the freight transportation industry which is frequently referred to as "containerization". With the advent of such cargo containers came the need for specialized vehicles adapted to efficiently handle and transport them between major freight transport facilities such as railroads, overland trucks, cargo airplanes and marine freighters.
It will be realized that when shipment involves passage on various types of such transporting means, transfer and transshipment of the cargo containers is necessitated, and various types of prior art machines for lifting and loading such cargo containers to and from one means of transport to another includes cranes and derricks of various forms, straddle lift van carriers, and lift trucks having specialized attachments.
A telescopic spreader frame for cargo container lifting is disclosed as an attachment to a lift truck in U.S. Pat. No. 3,764,032 and as an attachment to a van carrier in U.S. Pat. No. 3,874,719. One type of cylinder and piston assembly for actuating a telescopic container handling spreader frame to a plurality of selectable fixed positions is shown in U.S. Pat. No. 4,017,110. All of the above references disclose a spreader frame having twist lock latching mechanism located at each of its four corners for lifting cargo containers by corner fittings in the four upper corner castings of the cargo container.
Cargo container standards for the dimensions of the unit and location and configuration of the corner fitting have been developed by the International Organization of Standardization (ISO). These standards are frequently but not uniformly followed by cargo container manufacturers. For example, the widely used Sea-Land containers utilize corner fittings at different locations and with different corner fitting configurations than are specified by ISO standards. Universal twist lock mechanisms for a spreader frame for engagement with either ISO standard or Sea-Land cargo containers are disclosed in U.S. Pat. Nos. 3,749,438 assigned to Fruehauf Corporation, and 3,762,754 assigned to the Assignee of the present invention. Both of these twist locks as well as the twist lock used on the spreader frame for the Clark CY800 lift truck have some sort of device associated with them to indicate to the operator the fact that the twist lock has been inserted in the corner fitting and the fact that the twist lock has rotated. These systems are commonly called "interlock systems".
The interlock system used with U.S. Pat. No. 3,749,438 employs separate electrical sensors to signal to the operator that the twist lock has been engaged in the corner fitting and that the twist lock has rotated. Additionally, this system employs a mechanical linkage that connects the two twist locks of a lifting beam together so that the twist locks may be rotated with the activation of a single hydraulic cylinder.
The universal twist lock disclosed in U.S. Pat. No. 3,762,754 employs two shanks each having a hook member at its lower end. The hook members are positioned in different relationships for different corner fittings. This twist lock employs a complex hydraulically actuated linkage with electrical controls and interlock system.
The interlock system in the Clark CY800 spreader frame includes a pin spring biased to a position that prevents the rotation of the lower end of each twist lock. The pin is moved out of its interfering position when the twist lock engages the corner fitting of a container. Each shank is rotated by a separate hydraulic cylinder. This system can use a single lift-no lift electrical signal for each twist lock. The electrical signal is energized when the twist lock is rotated since the twist lock can not rotate until it is engaged with the corner fitting of the container. A disadvantage in this system is that a twisting moment is applied from the engaging hydraulic cylinders to the twist locks when the twist lock is in lifting engagement with respective corner fittings. The interlock system used in the CY800 spreader frame could not be used with the twist lock of U.S. Pat. No. 3,749,438 since this twist lock is free to pivot in its support and could pivot around the pin.